The present invention relates to ink jet printers, and more particularly to the driver for a piezoelectric crystal ink jet.
The typical driver for piezoelectric (PZT) crystal ink jets in the prior art consists of a linear high voltage amplifier which is generally D.C. coupled. This requires the use of components which can operate at high voltages (at least 250-500 volts peak-to-peak), and high voltage power supplies.
These drivers are high cost items and present a potential product safety problem. One factor is responsible for both of these; the high voltage requirements of the device. A linear amplifier which is capable of producing an output signal of up to 500 V.sub.p-p is an expensive item compared to a monolithic power amplifier which can produce an output signal in the 50 V.sub.p-p range. Additionally, these prior art drivers with their high voltage amplifier and high voltage power supplies create potential safety hazards.
An ink jet driver which uses a low cost, low power monolithic power amplifier with the higher voltages of the PZT only in the final stage is desirable. The present invention provides such a PZT driver.
In accordance with the illustrated embodiment, the present invention provides a low voltage PZT ink jet driver circuit for developing a high voltage PZT excitation signal in response to a low voltage video signal. The driver includes a power gain stage which is coupled to receive the low voltage video signal for amplifying that signal into a signal having a voltage level intermediate that of the input video signal and the necessary PZT excitation signal. Coupled to the output of the power gain stage is a serially connected capacitor-resistor network which in turn is coupled to the primary winding of a transformer means for stepping-up the video signal from the intermediate voltage level to the selected PZT excitation voltage across the secondary winding of the transformer means.
To maximize the power factor of the output power, the inductance of the secondary winding is matched to the average capacitance of the type of PZT ink jet selected to form a parallel resonant circuit therewith which has a resonant frequency at the selected droplet printing frequency. In turn then, the characteristics of the primary winding of the transformer means are set by the number of turns used in the secondary winding and the necessary turns ratio to obtain the desired voltage level in the excitation signal from the intermediate voltage levels of the video signal.
Additionally, the capacitor-resistor network and the primary winding form a series resonant circuit with the value of the capacitor selected to produce a resonant frequency that is substantially lower than the droplet frequency. The value of the resistor in the series resonant circuit is selected to minimize its Q and to critically damp or overdamp the series resonant circuit.